The news that Palestinian Prime Minister Mahmoud Abbas (search) has elected to step down from his post won't deter the United States from promoting peace in the region, according to an official statement released by the White House.

"We remain committed to implementation of the roadmap, working with Israelis, Palestinians, Arab States who seek peace," said the statement, referring to President Bush's vision for restoring order to the region.

But the statement also suggested that it was important to find someone else to fill the position — and soon. The creation of Abbas' job was a "key turning point for the Palestinian Authority in the development of new institutions to serve all the people, not just a corrupt few tainted by terror," said the statement, implying the change could exacerbate problems that have stood in the way of stability and democracy for the Palestinians.

The White House also cautioned both sides to "consider carefully the consequences of their actions."

Homeland Security Secretary Tom Ridge was less measured in his reaction, saying Abbas' decision "unfortunately, tragically will delay" the peace process.

Abbas tendered his resignation in a letter delivered to President Yasser Arafat (search), according to Palestinian negotiator Saeb Erekat.

Arafat hedged briefly before announcing he had accepted the offer and was ready to begin the delicate process of selecting a new co-chieftain. He will now have up to three weeks to name a replacement, during which time Abbas and his Cabinet will serve as caretakers of the government.

Just hours after Abbas' announcement, an Israeli warplane dropped a 550-pound bomb on a Gaza City apartment in a botched attempt to kill several top Hamas leaders, including the Islamic militant group's founder, Sheik Ahmed Yassin (search), who escaped with a minor injury.

Yassin was the highest-ranking Palestinian leader ever targeted by Israel, and top fugitives, including Mohammed Deif (search), No. 1 on Israel's wanted list, were also in the room, security officials told The Associated Press on condition of anonymity.

Israel saw Abbas' resignation as a power vacuum in which it felt compelled to act immediately against Hamas.

"No Hamas official is immune," said Israeli Foreign Ministry official Gideon Meir, adding that "there will be other chances" to go after Hamas leaders.

Reaction to the Palestinian prime minister's resignation has come from all over the globe. Britain's foreign secretary calls it a "grave" situation. He says the resignation "doesn't put the peace process back to square one, but it is a further difficulty."

The European Union says Abbas' move could have "serious repercussions" for peace. The organization has dispatched its foreign affairs chief to the region.

Meanwhile, Egypt's foreign minister says his government, which has been a key moderator in the conflict, would try to "help the Palestinian leadership end its crisis."

Saturday's announcement marked the latest development in an ongoing, seemingly never-ending power struggle between the two men, which began almost on the very day that Arafat —- under intense pressure —- reluctantly appointed Abbas to his post last April.

Since that time, Arafat has made it a point of publicly questioning Abbas' authority and his loyalty, according to Martin Indyk of the Brookings Institute. "He has done everything he could to undermine him," Indyk said.

Immediately after that announcement, Israeli Prime Minister Ariel Sharon's (search) office warned that it would not agree to any arrangement in which Arafat would head the government, with one senior official going as far as to demand that he be forced into exile.

"The state of Israel needs to ensure the security of its citizens, and the first step for that is expelling the terrorist Yasser Arafat," Israeli Cabinet Minister Danny Naveh said in a statement.

The latest standoff was over control of the security forces. Abbas demanded command over all men under arms, but Arafat refused to relinquish control over four of the eight security branches. And yet, there was still some debate as to whether Abbas' resignation would ever become final or if there was a chance he may return.

"I wouldn't totally rule it out, but I think we have to understand that he's not going to go back under the same circumstances," said Dennis Ross, who served as former President Clinton's top Middle East mediator.  "And I think the broader lesson is, if you can't get him to go back, you're not going to get anybody else to, either."

But not long after the news became public, Abbas, widely known in the region by his nickname of Abu Mazen, told parliament in a closed-door session that he would not change his mind. "Abu Mazen has made his decision," said Abdel Fatah Hamayel, a legislator from the ruling Fatah movement. "He's insisting it's a final decision.

Later in the day, Abbas released a statement in which he listed a number of reasons for his resignation, among them Israel's unwillingness to implement its obligations in the road map. He added that he felt the U.S. "did not exert sufficient influence on Israel" to carry out commitments under the plan, which calls for an end to violence and envisions creation of a Palestinian state by 2005.

Meanwhile, Arafat also said he had called meetings for Sunday with his Fatah faction to begin discussions about the leadership crisis. Lawmakers noted, however, that for Abbas' resignation to become official Arafat must first send him a "letter in writing," which had not yet happened.

Abbas supporters stressed that be had become increasingly more frustrated by the constant wrangling with Arafat. He was also said to have been hurt by the near-collapse of the U.S.-backed "road map" peace plan and his inability to improve the daily lives of Palestinians.

"There was great promise there, great hope there, but he was consistently being undermined by elements within the Palestinian Authority," Ridge said, speaking at a conference of political business leaders in the Italian town of Cernobbio.

Even if he hadn't resigned, Abbas might have been forced out. He faced a vote of confidence in parliament in the coming days, and there was growing dissatisfaction in parliament with his performance and his difficulties with Arafat.

Arafat's international standing could further be weakened if he were to be seen as having engineered Abbas' ouster. Abbas and Arafat have been at odds ever since Arafat appointed the prime minister under intense international pressure in April. The latest standoff is over control of the security forces. Abbas, backed by the United States, demands command over all men under arms, but Arafat refuses to relinquish control over four of the eight security branches.

The prime minister says he will not clamp down on militants, as required by the U.S.-backed "road map" peace plan. However, being in control of all the security forces would give him greater authority in renewed negotiations with Hamas, Islamic Jihad and renegades from his own Fatah movement.

Earlier this week, Abbas told parliament it must either back him or strip him of his post, saying he is not clinging to the job and would just as soon step down.

Both nations have said they will not do business with Arafat, whom they accuse of fomenting terrorism and consider an obstacle to peace making.

Israel's defense minister has threatened to expel Arafat. Israel's government has considered this idea in the past, but has been held back by U.S. opposition and by warnings of its security chiefs that Arafat could do more harm abroad than isolated at his West Bank headquarters.

However, the threshold for taking action against Arafat could be lowered by Abbas' departure.

The Associated Press contributed to this report.